This invention relates to permanent magnet synchronous motors, and more particularly to an improved rotor construction for two-pole permanent magnet rotors.
In some applications it has been found desirable to use motors of relatively small diameters, and which rotate in precise synchronism with an applied rotating field. Generally such small-diameter motors have usually been induction motors, it having been difficult in the past to construct true synchronous motors which are small, capable of rotating at high speeds, and having the necessary torque. Because of the small size and high speed of such motors, it has been difficult and prohibitively expensive to utilize segmented rotor construction and/or wire wound salient poles in the manner of ordinary synchronous motors.
In one approach to providing a rugged motor which exhibits synchronous characteristics, motor designers have turned to synchronous reluctance designs. Synchronous reluctance motors may be generally thought of as induction motors whose rotors have been provided with flux barriers which cause the magnetic reluctance of the rotor to vary with rotor orientation, in effect providing magnetic poles which "follow" the rotating magnetic field produced by the stator windings. Examples of such synchronous reluctance rotors are illustrated in U.S. Pat. No. 3,652,885--Honsinger and U.S. Pat. No. 3,862,446--Hilgeman et al. The flux barriers of these motors are constituted by axial slots extending through the stacked rotor laminations, which are filled with a nonmagnetic, high-reluctance material such as die cast aluminum. Such motors are typically provided with a set of axial rotor bar slots near the peripheries thereof, much in the manner of ordinary induction motors.
With the availability of high-strength permanent magnets, the cast flux barriers were replaced with magnets. With this approach, the magnets give rise to magnetic poles which interact with the stator flux to achieve synchronous motor operation. In effect, the permanent magnets take the place of electromagnetic windings of conventional synchronous motors. One example of such a construction is disclosed in U.S. Pat. No. 4,139,790--Steen.
Early construction of such permanent magnet motors closely followed synchronous reluctance motor layouts, by simply substituting blocks of magnetic material for die cast aluminum flux barriers. Subsequent investigations have shown that numerous design parameters must be modified in order to obtain permanent magnet motors which have the desired operating charcteristics while at the same time being relatively easy to manufacture, and strong enough to resist the forces arising during high speed operation. Because of the high speeds required of such motors the mechanical strength of the rotor laminations is of considerable importance, and must be maintained. Further, the pre-formed permanent magnets utilized for such rotors are ordinarily produced in the shape of rectangular bars which must be pressed into the rotor body, giving rise to exacting mechanical design considerations. In addition the rare earth magnets which are now often used in such rotors are very expensive, which makes it desirable to limit the number and volume of the magnets. On the other hand, it is desirable to array the magnets in a manner which provides the maximum magnetic area for each pole. Still another consideration is the cost of the magnets, which makes it desirable to utilize magnets of a common size and shape.
It will therefore be appreciated that it would be highly desirable to provide an improved two-pole permanent magnet rotor which is mechanically strong, while using less magnetic material and exhibiting better performance characteristics than those previously known.
Accordingly, it is an object of the invention to provide an improved two-pole rotor for a permanent magnet synchronous motor.
Another object is to provide a two-pole permanent magnet synchronous motor rotor which exhibits improved torque charcteristics and makes better use of magnetic materials than those herein before known.
Yet another object is to achieve an advantageous rotor design utilizing magnets of a common size and shape.